Mexico City is rebranding itself ... as Mexico City. In English, that sounds bizarre. But, for nearly 200 years, the capital's official name has been Distrito Federal, emphasizing its status as the nation's political center. That changed this January, when city leaders announced they were changing the city's name to reflect a wave of changes giving the supermetropolis more autonomy. So, Cuidad de Mexico. Or, per the new logo: CDMX.

The branding is especially thick in the city's museum and monument-filled central district. Especially around the towering Hilton Reforma Hotel. And especially during the last week of November, when the hotel hosted the C40 Mayors Summit, a conference focused on how cities are fighting climate change. Regardless of what their home nations are doing—in spite of Trump's election, Brexit, and other geopolitical spasms—many of them are pushing forward.

So, when Andrew Steer, president of the World Resources Institute, says "the battle for climate change will be won or lost in the cities of the world" to a packed conference room at the C40 summit, he's only partly playing to his audience. His declaration came after C40's executive director Mark Watts1 had presented the findings from Deadline2020, a 100-plus-page C40 study detailing how the organization's 90 affiliated cities can, and are, taking rapid, impactful actions to keep the Earth from warming to the point of catastrophe. As Watts explained—his speech punctuated by a jarring tick-tock noise playing over the PA—these cities need to peak their emissions by 2020.

Deadline2020 is based on goals set out in the Paris Agreement, a UN treaty signed by (as of this writing) 117 countries to keep the average global temperature from rising to 2˚C above pre-Industrial levels. Actually, the Paris Agreement calls for that temperature rise—which is already happening, by the way—to stay as close to 1.5˚C as possible. Deadline2020 aims for that more ambitious goal.

In that frame, the next four years are critical. Climate scientists talk about "locked-in warming." This is warming that is already destined to happen because of present emissions levels. If the C40 cities want to meet that 1.5˚C goal, its members need to collectively cut the average emissions of city-dwelling folks from 5.1 tons to 2.1 per person by 2020. (According to C40's own assessment of the atmosphere's current inventory of greenhouse gases, the world is already locked-in for 1.2˚C of warming. Other, more pessimistic, studies say the 1.5˚C goal is already blown.)

If they succeed, the cities of C40 will have contributed 40 percent of the reductions necessary to meet the most ambitious goal of the Paris Agreement. Come hell or high water, their home nations are largely responsible for the other 60 percent. So in that frame, "the battle for climate change will be won or lost in the cities of the world," isn't entirely true.

Still, as author William Sweet writes in his new book Climate Diplomacy from Rio to Paris: "If you are in a car hurtling toward a cliff or brick wall ... You slam on the brakes and hope for the best."

One Big Battle

Climate change-fighting actions always come down to two questions:

How much can you do?

How fast can you do it?

Those metrics are usually inversely related. Depending on income, an individual person can immediately take steps to fight climate change: Buy a Tesla, install solar panels, recycle a cardboard box, install double-paned windows, plant a tree. None of those actions, obviously, comes close to saving the world.

At the other end of the scale are nations, which can create society-wide changes to reduce greenhouse gas emissions. But, time wise...

Just look at the pace of climate diplomacy. The UN first agreed that climate change was dangerous in 1992, at the Rio de Janeiro summit. It took nearly two whole decades for them to agree upon a numerical value for how much global warming counted as dangerous—2˚C, in Copenhagen—and another six before signing an agreement to take action before the world reaches such dangerous warming (the aforementioned Paris Agreement, ratified this September in New York).

And while many countries have made commitments to cut their emissions—the US released the Clean Power Plan to cut coal, China is rolling out cap and trade, India intends to build the world's largest solar project—they still haven't agreed on how to do basic stuff, like agree on a uniform, independently verifiable way of counting how much each country currently emits.

And that's not to mention the Paris Agreement's uncertain future. President-elect Donald Trump has sworn to withdraw the US's commitment to the international coalition, and kill the Clean Power Plan. That doesn't necessarily knife the agreement, but it certainly damages the good faith that negotiators, US and abroad, spent years building in order to get the Paris Agreement to the table.

Cities can't save the Paris Agreement, or the world, on their own. But they can do a lot. And they can do it a lot faster, and a lot more reliably, than nations. Look at Mexico City. In 1992, it was the most polluted city on the planet. For the week of the C40 conference, the air quality was in the moderate zone—not great, but not the worst. This is thanks to laws limiting when drivers can drive, regulations forcing its taxi and bus fleets to go green, and an emphasis on making streets and neighborhoods more bike and pedestrian-friendly. All of those things, of course, also lower the city's carbon footprint.

And cities' environmental decisions don't happen in a void. Urban areas are big economic players—Mexico City contributes around 16 percent of Mexico's annual GDP—and their investments send ripples through global markets. Another example: China's Shenzhen has begun electrifying its fleet of taxis and buses, and plans to convert private vehicles as well. That's 15 million people whose daily travels won't create a demand for oil.

C40 Cities came about after then-mayor of London Ken Livingstone convened mayors from 17 other cities that were already taking action on climate action. He envisioned a network in which metropoli could optimize their individual efforts by sharing ideas, data, and mistakes. Further, he and the other mayors thought the group should include 20 cities from the northern hemisphere, and 20 from the south—the 40 in C40. It's since grown. Members include the obvious (Portland), the influential (New York), the sprawling (Johannesburg), and the growing (Addis Ababa). Between 2005 and 2016, they have taken more than 11,000 total actions to emit less, mitigate the stuff that is emitted, and fortify themselves against threats like higher tides, worse floods, and longer droughts.

Those actions aren't always sexy, but they are tangible. Mexico City's double-articulated MetroBuses—which move over a million people each day, keeping tens of thousands of tons of greenhouse gases out of the atmosphere—are a rare ooh-worthy example.

But many more are projects only a city planner could love. At the C40 summit, a ceremony held in the neoclassical courtyard of the Palacio de Minería celebrated the best sustainability projects from the previous year: Kolkata received thunderous applause for getting 60 to 80 percent better at sorting its solid waste; Sydney and Melbourne took a joint bow for prioritizing green building codes; Copenhagen earned its ovation for tackling its intermittent flooding issues (due to both sea level rise and heavy rains) by installing what are essentially grass strips along its curbs.

Individually, each of these actions amount to decimal points against the total number of cuts needed to keep the Earth from warming 1.5˚C. In aggregate, they do a lot. Especially along with the 11,000 other actions taken since 2005. According to the Deadline2020 report, the C40 cities need about 14,000 new actions before 2020 to do their part in averting catastrophic warming.

Why Cities?

I am cynical. So as I went from meeting to interview to plenary to more meetings at C40, I was bothered by a number mentioned in the Deadline 2020 report. It estimates that C40 cities will collectively invest more than $375 billion between now and 2020 on those 14,000 remaining actions. I could not help but wonder what was in it for the cities.

When I posed this question to Clover Moore, Lord Mayor of Sydney, she told me that cities have to spend huge amounts of money anyway, and often the green solutions are better for the city's bottom line—transportation projects don't just move people, they move people's money. Greener cities also attract demographics that raise a city's profile: millennials, creative class, young professionals; pick your buzzword, they like clean cities, and cities like them.

So, that sort of answers the why, but not the how. How do cities not get bogged down in the political divisiveness that has historically stalled climate action? “In higher levels of government, oftentimes the debates are philosophical," Muriel Bowser, mayor of Washington, DC, told me during a sit-down chat between plenaries. Nations are big enough, and their economies diverse enough, that their elected officials can exhaust years bickering over the science. "A mayor doesn't have that as an option," says Bowser. "You have to run the transportation systems, pick up trash, make sure kids get to school, keep the police policing, and firefighters fighting fires.” Climate change is already pressuring many of those civil services. Cities demand action.

And not only have these cities spent over a decade trading ideas (and mistakes), they have already gotten their member cities to agree on a standardized emissions accounting system: the Global Protocol for Community-Scale Greenhouse Gas Emission Inventories (GPC). Nearly every C40 city reports its emissions using this standard. “The collection of carbon data is fundamental to everything we do at C40," says Thomas Bailey, an energy analyst with the organization.

How Cities Count Emissions
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Start with sourcing the carbon inputs and outputs for every bit of infrastructure, every component of public works. Let's say you're looking at a subway. The energy to run it might come from a mix of different power plants, each with different emissions profiles. Done? Okay, now calculate ridership. "So say we know there are 30 million subway trips each month, and subtract that from the estimated number fuel sales that would have required,” says Seth Schultz, lead analyst for C40.

That's the easy part. After that you have to standardize the system for every city, each of which has a different bureaucracy. For instance, one city’s waste department might keep track of the trucks transporting its waste, where another city does so under the transportation department. Now you've got a system for tracking greenhouse gas emissions.

There's another facet to the "why" of cities taking the lead on climate action. But first, quick question: Can you name, without calling on Google, the strongest hurricane ever recorded? (Hint: It happened last year.) No? Probably because it (Hurricane Patricia, with high winds of 215 miles per hour) made landfall in a sparsely populated part of Mexico, killing nobody.

Mexico City is filled with mementos of disaster. About a mile down the road from the Hilton Reforma is the gigantic Metropolitan Cathedral. Behind it, there used to be a large, block-size lump that locals called "island of the dogs," after the strays that would gather there when storms flooded the surrounding streets. In the late 1970s, electrical workers were digging new lines through the lump when they hit something big: a 10 and a half foot diameter stone disk, depicting the dismembered Aztec goddess Coyolxauhqui. Over the next several years, archaeologists excavated the site to reveal the Templo Mayor, a major Aztec religious complex with a double-headed pyramid as its centerpiece.

Before Ciudad de Mexico, before Distrito Federal, before Hernán Cortés tore down the Templo Mayor and everything around it, this place was Tenochtitlan, capital of the Aztec civilization. Mexico City's living inhabitants are regularly confronted with reminders of what their city once was. Cortés and climate change are different kinds of disasters, but every city eventually gets turned into something else. Some are lucky enough to choose what they become.

1UPDATE: 12/16/2016 12:40pm — This article originally omitted Mark Watts' role in presenting the Deadline2020 report.